Electrical properties of metal/GaN (M-S) and insulator/GaN (I-S) interfaces have been investigated for the fabrication of MIS Field Effect Transistors.1. Characterization of M-S interfaces; (1) Fermi level at metal/GaN interface is unpinned, and the Schottky barrier height (SBH) is predominantly determined by the metal work function. (2) Reported large discrepancies in the SBH's between I-V and C-V measurements, scattered Richardson constants, temperature-dependent SBH, and appearance of I-V shoulder are well explained by additional leakage current arising from small "patchy" defect regions with low SBH's. (3) The sum of n- and p-GaN SBH's at room-temperature is rather small as compared with the GaN band gap while it becomes close to the GaN band gap at high-temperatures. These phenomena can be again explained by the proposed "patchy" model.2. Characterization of I-S interfaces; C-V measurements of Al/PCVD-SiOィイD22ィエD2/n-GaN structures revealed that the interface Fermi level locates at around 0.3 eV from the conduction band edge under thermal equilibrium condition and it can move within the upper forbidden band gap with the applied gate bias, indicating the feasibility of MISFET devices. A relatively small band bending for n-GaN surface was also confirmed from C-t measurement.3. Improvement of M-S and I-S interfaces; The uniformity of the SBH is greatly improved by an annealing in NィイD22ィエD2 (400-500℃), without any degradation of the Schottky characteristics. The observed I-V shoulder was considerably reduced with a fairly good n-value. For the I-S interface, a brief annealing in HィイD22ィエD2 (300-500℃) is highly effective to improve the interface properties, and the minimum interface state density is reduced below 1 X 10ィイD111ィエD1 cmィイD1-2ィエD1eVィイD1-1ィエD1